Survey shows billions of juvenile Arctic cod under sea ice
Arctic cod. Photo: Robin West/U.S. Fish and Wildlife Service.What happens under the Arctic ice cap has largely been hidden from the prying eyes of science. But a new under-ice trawl net has changed that. A recent study published in the journal Polar Biology found a Arctic cod lurking there in the billions.Download AudioImperiled habitat. That’s what you might call your digs if you live under the ice in a warming ocean.A new study by German researchers estimates the prevalence of nearly nine billion Arctic cod in the eastern Arctic Ocean alone. They’re a species that spends substantial time under the ice.Nine billion sounds like a lot, but University of Alaska fisheries scientist Franz Mueter, who’s also surveyed for Arctic cod, says they’ve likewise found them by the billions living in the Chukchi. But,“We didn’t sample under the ice like they did,” he says. “And that’s the novel thing here, is that they sampled under the ice in the central Arctic which hasn’t really been done much before.”The special net they used, developed in the Netherlands, sinks below the sea ice, and then inflatable floats bring it to the surface. Instead of trawling the bottom of the ocean, the net scrapes where ice meets ocean.The Surface and Under-Ice Trawl (SUIT) equipment was developed at the IMARES research institute. Photo: Screen grab from YouTube demo video.The survey didn’t sample from under multiyear ice, but where they did go is still new territory for science.Before the net, most surveying for Arctic cod was conducted in open seas. And that misses a key part of the species’ life cycle.Arctic cod spawn under the ice; they actually lay their eggs there. And below the ice, juveniles find a nursery of sorts. There’s plenty of zooplankton to eat. There’s shelter from predators.The study found 1- and 2-year-old fish under the ice, which was sort of surprising. In other parts of the Arctic Ocean, fish this age have already moved to the ocean floor. Mueter again:“And these (cod) presumably stayed with the ice as the ice retreats, either because it’s being pushed back to the north or because it melts and they just move further under the ice that hasn’t melted yet.”So under the sea ice: it’s a good place to be a baby fish.And that’s important for an abundant little fish that shoulders the northern food chain:“Arctic cod is probably the central, or the most important, prey species in the Arctic,” Mueter says.Sea birds, belugas, narwhals,seals — Arctic cod is a preferred food source for all those animals.“Indirectly they’re quite important to subsistence hunters that depend on (marine) mammals,” Mueter adds.There’s not a commercial fishery for Arctic cod (except for a small one in Russia). Adult fish are only about 10 inches long. But there is concern that an increasingly ice-free Arctic could lure fishermen north, especially as more traditional commercially fished species are likewise migrating north in search of cooler waters.Earlier this summer Arctic nations agreed to a tenuous ban on unregulated fishing in the high Arctic — that is, until more surveys like this one can better assess fish stocks so they can be responsibly regulated.Note: Most the world calls this fish ‘polar cod,’ but ‘Arctic cod’ is most commonly used in Alaska.